Review of characteristics of mercury speciation and mobility from areas of mercury mining in semi-arid environments

Abstract

The speciation of mercury-including most phase minerals, secondary phases, gaseous and aqueous species-is very important for evaluating the environmental impact and mobilization of this contaminant. Mining activities produce mercury mine waste, which includes several types of material (mainly mine waste and calcines) with varying mercury content and speciation depending on the ore deposit and processing technology. The main phase minerals are cinnabar, metacinnabar, metallic Hg 0 , corderoite, livingstonite, calomel and schuetteite. The aqueous mobilization of mercury is controlled by complex formation, pH-Eh conditions, the primary phase mineral of mercury, and organicmatter and iron oxyhydroxide content. The possibility of colloidal transport of mercury from mine waste is influenced by the atmospheric emission of metallic Hg 0 and the leaching of waste by episodic highintensity precipitations. In these climatic conditions, mercury can be mobilized to pore water, surface water or groundwater by the dissolution of metallic Hg 0 and cinnabar in acidic conditions, and by the colloidal transport. The presence of Hg-soluble phases (chlorides and oxychlorides) may enhance the mobilization of mercury. In the semi-arid conditions of the Valle del Azogue (SE Spain) the atmospheric emissions of the Hg 0 present in calcines and mine waste may be significant and the dissolution of Hg 0 and metal-sulfate salts during episodic runoff events may explain the mobilization of Hg, Fe, Pb, Zn and other heavy metals.

Mercury Pollution Assessment of Mining Wastes and Soils From Former Silver Amalgamation Area in North-Central Mexico

“…The Hg degassing rates depend on multiple climatic factors, such as temperature, solar radiation and rainfall events (Song andVan Heyst 2005, Moore andCastro 2012). Therefore, the semiarid conditions at the study site potentially promoted the release of Hg from the surfaces of the mining waste dumps (Navarro 2008). Figure 3 shows the total Hg concentrations in soils at the Cedral sampling sites, which ranged from 1.0 mg/kg to 116 mg/kg and were lower than the concentrations observed in the mining waste sample areas.…”

“…Thus, the high Hg concentrations in the soils at the sample sites in Cedral primarily resulted from the direct impacts of the amalgamation process and the mining waste deposition. However, the dispersion of Hg particles by wind and/or water transport also enriched the soils, and these dispersion mechanisms are influenced by climate and topography conditions (Navarro 2008, Santos-Francés et al 2011). The potential contributions of dispersion as a pollution mechanism in the study area were observed at sample sites NAS-1 and NAS-2, which were outside of the former amalgamation facilities.…”

“…Thus, the distribution of Hg in the soil profiles suggested that physical and chemical Hg transport processes affected the downward movement of Hg through the soil profile. The mobilization of Hg in soils is controlled by several factors, including Hg speciation, soils physical and chemical properties and climatic conditions, such as precipitation and evaporation (Navarro 2008).…”

“…Therefore the low solubility of Hg in the mining waste and soil samples under simulated rainwater contact conditions indicated that aqueous transport was a minor contributor to the vertical migration governed by soluble chemical Hg species. Thus, colloidal transport could play a major role in the downward migration of Hg in soils (Navarro 2008).…”

“…The latter because the predominant chemical species associated with the residual fraction are most likely elemental Hg and HgS (Lacerda 1997, Johnson and Whittle 1999, Lang 1999. However, elemental Hg is subjected to chemical transformations, such as oxidation, dissolution, amalgamation, precipitation and adsorption, which affect its chemical speciation during the amalgamation process (Slowey et al 2005) and its transformations due to chemical weathering or biological activity (Navarro 2008). Moreover, the chemical transformations of Hg that occur when amalgamation mining wastes are reprocessed remain unknown.…”